Plasma is highly ionized gas containing an approximately equal number of positive ions and electrons.
Plasma sources for industrial application have been developed during the last 30-35 years. At the present time plasma sources find wide application in various fields of technology, in particular in the manufacture of semiconductor devices, e.g., for cleaning, etching, deposition, etc., in the production of semiconductor chips.
There exist a great variety of plasma sources that differ in the methods of plasma excitation and the geometry of the electrodes and plasma volume, which, in turn, determine major parameters of the plasma.
Plasma sources with parallel electrodes (so called "diode systems"), with parallel electrodes and a grid between the electrodes (so called "triode systems"), and magnetron-type plasma sources have found the widest application in the industry. In these sources, plasma may be generated by direct current (DC) power or by a high--frequency (RF frequency or higher, up to microwave [MW]) power source.
A typical diode plasma source is described by J. Reece Roth in: Industrial Plasma Engineering, Vol. 1, Principles, Institute of Physics Publishing, Bristol and Philadelphia, 1995, pp. 332, 438. Such a source consists of a vacuum chamber containing two parallel-plate electrodes. One electrode is grounded, and the other is connected to a negative terminal of a power supply source, e.g., an RF power supply. An object to be treated, e.g., a semiconductor wafer, is attached to the powered electrode. A working gas is supplied to the vacuum chamber, an RF power supply is activated and generates a plasma between the electrodes. This plasma has a certain spatial distribution with respect to the electrodes and the wafer. In such configuration, the surface of the wafer can be treated by energetic ions extracted from the plasma by the powered electrode and accelerated in the space between the plasma boundary and the surface of the wafer.
When energetic ions are not required, e.g., for etching by free radicals, by atoms in excited state, or by other plasma-related species, the wafer can be attached to the grounded electrode, while the other electrode is connected to a power source, e.g., an RF power supply.
in a triode-type plasma source, the grid is used for improving control of the ion energy on the surface of the object being treated. This is achieved by applying a control voltage to the grid.
However, the parallel-plate plasma source has a common disadvantage consisting in a low efficiency of ionization which does not allow to reach the ion density higher than 10.sup.10 cm.sup.-3. Another drawback of such plasma sources is inability to control plasma space distribution and flow of charged particles.
Further development of plasma technique led to the design of low-pressure high plasma density sources, such electron cyclotron resonance plasma sources (hereinafter referred to as ECR plasma sources) and helicon plasma sources, which are described below.
A typical ECR plasma source is described, e.g., by Donal L. Smith in: Thin-Film Deposition. Principles and Practice, McGraw-Hills, Inc., NY-Toronto, 1995, p. 511.
It comprises a discharge chamber made of a nonmagnetic material with a quartz window for the supply of MW power for generation of plasma and a pair of solenoids which embrace the discharge chamber and are intended for generating a magnetic field in the gas-discharge chamber which creates conditions required for generating electron cyclotron resonance on a frequency of electromagnetic radiation of the microwave source (2.45 GHz). A working medium is supplied to the discharge chamber via a working gas supply tube for the supply of a working gas to the discharge chamber.
The aforementioned plasma source operates as follows:
The discharge chamber is evacuated to a high degree of vacuum of about 10.sup.-3 to 10.sup.-5 Torr required for effective absorption of microwave energy by the electrons of the plasma under conditions of the cyclotron resonance, and a working gas is supplied to the discharge chamber. A magnetic field with the intensity of about 87.5 mT is induced in the discharge chamber by the solenoid. A MW power with the frequency of 2.45 GHz is introduced into the discharge chamber via the quartz window. This generates a gas-discharge plasma. Since the coefficient of plasma diffusion in the transverse direction of the magnetic field several ten times smaller than the coefficient of plasma diffusion along the lines of forces of the magnetic field, the obtained plasma diffuses along the lines of forces of the magnetic field towards an object to be treated, e.g., a semiconductor substrate. Configuration of the plasma can be controlled by means of an additional solenoid located in the vicinity of the substrate.
A disadvantage of the aforementioned ECR plasma source consists in that it is difficult to control distribution of the plasma concentration from the source axis to the source periphery. This is because the conditions of the optimum discharge and the specific distribution of lines of forces of the magnetic field are interrelated. More specifically, the plasma is formed mainly in the area of equality between the frequency of rotation of electrons in the magnetic field and the frequency of the MW power source [see aforementioned reference to Smith, p. 511, 512]). Therefore, displacement of the plasma formation zone under the effect of the solenoid located in the vicinity of the substrate will affect the distribution of line of forces of the magnetic field and, hence, density of the plasma near the substrate surface and thus uniformity of treatment.
Other drawbacks of the aforementioned ECR plasma sources are dependence of the plasma formation zones on the zone of plasma diffusion in the direction of lines of forces of the magnetic field, a complicated construction of an ECR source which requires the use of an MV generator, necessity of matching of this generator with impedance of the gas-discharge plasma, a complicated construction of the gas-discharge chamber, etc.
Another disadvantage of the ECR plasma sources is that they are not suitable for uniformly treating objects of a very large surface area. This is because with an increase in the size of an object being treated, it is necessary to increase the diameter of a gas-discharge chamber, whereby uniformity of plasma across the cross-section of the gas-discharge plasma will be reduced.
Known in the art also are inductance plasma coupling (ICP) sources described, e.g., in the aforementioned book of J. Reece Roth, p. 413. They differ from the ECR plasma sources by the fact that plasma is excited by an electromagnetic field, e.g., of a flat spiral coil placed onto one of the electrodes. The power supply may be obtained, e.g., from a 13.56 MHz power source. These sources operate under pressures of 10.sup.-3 to 2.multidot.10.sup.-2 Torr. and, similar to the ECR source, here the plasma density may reach 10.sup.12 cm.sup.-3. ICP sources suffer from the same disadvantages as all other known plasma sources, i.e., difficulty to control the plasma density distribution, etc. Furthermore, ICP of large diameters require the use of high currents for plasma excitation coils which consume high power. This, in turn, makes the construction of the plasma source complicated and expensive, e.g., due to the use of a developed cooling system.
Another known source of the gas-discharge plasma is a so-called Penning discharge source [see aforementioned reference to Roth, p. 204.] The Penning discharge is a discharge that occurs in a longitudinal magnetic field between a cathode and an anticathode (i.e., an electrode through which plasma flows into a vacuum chamber, i.e., toward the substrate, and which has a cathode potential). As shown in FIG. 1, which is a schematic sectional view of a known Penning discharge source 20, the source has a tubular anode 22 located inside a gas-discharge chamber 24 between a cathode 26 and an anti-cathode 28. Anti-cathode 28 has a number of openings for extraction of ions from a plasma P generated inside cylindrical anode 22.
An object to be treated, e.g., a semiconductor substrate OB, is placed in a vacuum chamber 30 which is sealingly connected to plasma source 20. Anode 22 is connected to a positive terminal 32a of a DC power source 32. A negative terminal 32b of power source 32 is grounded at G1. Cathode 26 is grounded at G2 via a conductor 34 that passes through the wall of gas-discharge chamber 24 via an electric feedthrough 36. Anti-cathode 28 is grounded at G3 via a conductor 38 that passes through the wall of vacuum chamber 30 via an electric feedthrough 40.
In the Penning discharge, electrons perform oscillating movement between cathode 26 and anti-cathode 28, traveling along helical trajectories along lines of forces of the magnetic field.
The working gas is ionized under pressures within the range of 10.sup.-1 Torr to 10.sup.-7 Torr at a magnetic induction of 100 to 3.multidot.10.sup.4 Ga. Discharge voltage can varv between 100 and 50,000 V, and the current can vary from 10.sup.-7 to 20 A. Temperature of electrons is within the range of 2 to 15 eV, while ion energy may vary from 1 eV to several keV.
The wide range of possible changes in the parameters of the discharge makes Penning discharge sources suitable for efficient industrial application in various fields. Nevertheless, they are unsuitable for uniform treatment of objects of a large surface area, since they cannot provide uniform distribution of plasma concentration in a radial (transverse) direction of plasma, especially in the case of a large cross-section of the plasma required for treating substrates having diameters up to 300 mm. Another disadvantage of such sources is dependence of a plasma flow distribution on the ionization of the working gas. In spite of all the advantages of the Penning discharge sources, the last-mentioned drawback significantly limits the scope of their practical industrial application, especially in view of latest rapid development of the electronic industry.